World's oldest marsupial found in China

This ancient marsupial probably lived near fresh water (Science/Mark A Klingler/CMNH)

An exquisitely preserved 125 million-year-old fossil found in China has set a new record for the oldest known marsupial and may rewrite the history of mammal evolution, a new report suggests.

It beats the previous record-holder from North America by 25 million years and suggests that vital steps in mammal evolution took place instead in Asia, according to a team of Chinese and American scientists reporting in today's issue of the journal Science.

"The oldest placental mammal came out of the same Chinese rock formation, so Americans will now have to swallow hard and acknowledge that it would appear the whole thing got going in Asia," commented palaeontologist Professor Michael Archer, director of the Australian Museum in Sydney.

"So much field work is happening on this question around the world now that we're getting a picture of mammal evolution that simply wouldn't have been possible to even sketch only 20 years ago."

Archer said he wanted to know more details about the fossil to be sure it was a marsupial, but the evidence seemed convincing.

The mouse-sized fossil, complete with hair and most of its skeleton, was found in the Yixian Formation in western Liaoning Province. Its age makes it contemporary of the dinosaurs.

Its wrist and ankle bones, and some of its teeth, suggest it was a primitive marsupial, the group of mammals that raise their young in external pouches and includes kangaroos, wombats, bandicoots and opossums.

This one, called Sinodelphys szalayi, probably lived in bushy vegetation near fresh water. It probably ate insects and worms and had a body suited to climbing the lower branches of trees and shrubs, the report said.

"This mammal could be the great grand aunt or uncle, or it could be the great grandparent of all marsupial mammals," said Dr Zhe-Xi Luo, curator of vertebrate palaeontology at Carnegie Museum of Natural History in the U.S. and lead author of the report.

The Yixian formation has produced many startling fossils, including Eomaia scansoria, which was found last year and is thought to be the oldest placental mammal, also about 125 million years old. Researchers also found the fossil Archaeofructis, the world's oldest flowering plant, there.

Ancient mammals

Mammals as a whole have an even more ancient lineage, with the oldest known fossils being found in Texas and dating back at least 225 million years.

It had been thought that placental mammals evolved first and that marsupials were a later offshoot from that family tree, with North America being the likely point of origin for both groups, Archer said.

Now that both the oldest placental and marsupial mammals have been found in China, that assumption needed to be revised: "This find puts Asia right at the centre of interest when it comes to the evolution of mammals. It also means that marsupials can proudly hold their pouches up high in terms of their age in relation to placentals."

Fossils with marsupial-like teeth from about 100 million years ago had been found in Texas and the oldest confirmed early marsupial skeleton, aged 75 million years, was found in Mongolia.

The new find should help scientists learn when placentals and marsupials diverged from each other, but it now seems almost certain that the marsupials arose first in the northern hemisphere.

Yet the 270 living marsupial species - mainly found in Australia, New Guinea and South America, plus the North American opossum - are now found overwhelmingly in the Southern Hemisphere. They are the second most diverse mammal group after the placentals, which includes humans, and boasts 4300 species.

Marsupials once dominated the world in prehistoric times, Archer said. They appear to have reached the southern hemisphere through the South American section of the ancient supercontinent of Gondwana. Fossil records confirmed that they crossed Antarctica in warmer times and into Australia.

The oldest Australian marsupial fossil yet found, at Murgon, in Queensland, is 55 million years old.

But something happened about 15 million years ago, possibly a global change in climate, that caused the marsupials to disappear from North America, Asia, Europe and Africa. The opossum appears to have re-entered North America from South America more recently.